1. Field of the Invention
The present invention relates to a light emitting device for emitting visible light using a compound semiconductor material of AlGaInP group or AlGaAs group, or more in particular to semiconductor light emitting device capable of preventing the reduction in luminance which otherwise might be caused by the distortion due to the difference of lattice constant between the semiconductor layers in a multilayer structure.
2. Description of the Prior Art
The conventional semiconductor light emitting device for emitting visible light uses a compound semiconductor material of AlGaInP group, for example, for a emitting layer forming portion, and has a structure as shown in FIG. 4. Specifically, in FIG. 4, an n-type clad layer 22 made of a semiconductor material of n-type AlGaInP group, an active layer 23 of a non-doped semiconductor material of AlGaInP group having a composition lower in band gap energy than the clad layer, and a p-type clad layer 24 made of a semiconductor material of p-type AlGaInP group are formed by epitaxial growth on a semiconductor substrate 21 of n-type GaAs, thereby forming a emitting layer forming portion 29 of a double hetero-junction structure. Further, a p-type current diffusion layer 25 made of GaP is sequentially formed by epitaxial growth on the surface of the emitting layer forming portion 29. Further, a p-side electrode 27 of Au--Ge--Ni alloy or the like is formed on the front surface of the p-type current diffusion layer 25, and an n-side electrode 28 of Au--Ge--Ni alloy or the like is formed on the reverse side of the semiconductor substrate 21.
In the light emitting device of this structure, the light from the front surface of the semiconductor multilayers, i.e. the light from the side of the p-side electrode 27 is utilized, and the p-side electrode 27 which blocks the light is formed as small as possible in an area. In order to emit light by containing carriers in the active layer 23 sandwitched between the clad layers 22, 24, on the other hand, the current desirably flows distributed over the entire active layer. For this purpose, the current diffusion layer 25 is formed as thick as about 10 to 70 .mu.m to expand the current over the entire chip. This current diffusion layer 25 desirably not only diffuses the current but also fails to absorb the light emitted by the active layer 23, and is made of GaP which is large in band gap energy. Also, the p-type layer 24, the only function of which is to contain the carriers, is normally formed to the thickness of about 0.5 .mu.m not to absorb light.
In the conventional semiconductor light emitting device having the structure shown in FIG. 4, the current diffusion layer, which diffuses the current, is required to be formed as thick as possible. However, the lattice constant of the current diffusion layer of GaP and that of the compound semiconductor layer of AlGaInP group or AlGaAs group used for the emitting layer forming portion are considerably different from each other, and therefore a distortion is liable to be caused by the lattice mismatch. Specifically, although the matching can be secured between the GaAs layer of the substrate and the emitting layer forming portion for forming an emitting layer, the matching between the emitting layer forming portion and the GaP layer is difficult to secure. The present inventors have vigorously studied a method of improving the luminance of the semiconductor light emitting device of this type. As a result, the inventors have discovered that the luminance is extremely reduced if the distortion due to the lattice mismatch affects the active layer constituting the emitting layer.